I'd imagine they've done the studies and found that they'd lose more money overall. The gain in new customers who only want a specific channel must not outweigh the revenue they'd lose by customers downgrading to less channels.

It's also possible that it would produce the secondary effect of killing channels that are more niche and not quite as profitable (maybe not profitable at all). Maybe there are channels that people would never pay for individually if given the choice, but would watch if given them as part of a package, and the aggregate value of which provides enough additional benefit to justify paying for the package as whole.